Addition of Ni is effective in improving fracture toughness at a low temperature. For example, Patent Literature 1, Patent Literature 2, and Patent Literature 3 disclose a so-called 9% Ni steel (steel material containing Ni of about 8.5-9.5% by mass, having a tempered martensite structure, and mainly having excellent low-temperature toughness, for example, exhibiting excellent Charpy impact absorbing energy at −196° C.) as a type of steel used for an inner bath of a liquefied natural gas (LNG) tank.
Further, for example, Patent Literature 4 and Patent Literature 5 disclose a steel material containing Ni of about 4.0%, mainly having a tempered martensite structure, and having excellent low-temperature toughness, for example, exhibiting excellent Charpy impact absorbing energy at −70° C. as a type of steel for use in a ship.
While the low-temperature toughness can be improved by adding Ni, Ni segregates in the steel at the time of casting, and low-toughness structures are locally generated, possibly leading to a decrease in toughness in a weld heat-affected zone. Several methods for improving toughness have been proposed. For example, Patent Literature 6 discloses a method of performing a preliminary heat treatment for reducing the segregation before a casting slab is heated and rolled. Further, Patent Literature 7 discloses a method for reducing defects at a plate thickness center by dividing the rolling process into two processes. However, with the method disclosed in Patent Literature 6, the segregation reduction effect is not sufficient, and hence, a band-like Ni segregation remains, which reduces the toughness in the weld heat-affected zone. With the method disclosed in Patent Literature 7, a reduction ratio (thickness reduction ratio) from the casting slab to a final plate thickness (the reduction ratio is a value obtained by dividing a plate thickness before the rolling by a plate thickness after the rolling) is small, and the reduction ratio of a first hot rolling and temperatures are not controlled. Therefore, toughness of a base material and weld heat-affected zone decreases due to coarsening of the structure and the remaining segregation.
Further, Patent Literature 8 discloses a method using a TMCP (Thermomechanical Controlled Processing) in which water cooling is performed immediately after the rolling process, in order to manufacture a steel material having excellent toughness in a weld heat-affected zone. However, in a case where a low-temperature rolling is strengthened by using the TMCP, strength anisotropy becomes large, which causes a safety problem.
That is, it is difficult for the existing technique to manufacture a steel material that exhibits excellent low-temperature toughness in a base material and a weld heat-affected zone and has small strength anisotropy by using a steel material containing Ni.